The invention relates to a noise detector for detecting noise pulses in a non-limited radio receiving signal modulated on a carrier, comprising an amplitude detection circuit and coupled thereto a signal comparison circuit having a signal input and a threshold input. The signal comparison circuit produces a detection signal when the signal at the signal input exceeds a threshold level which depends on a threshold signal at the threshold input, this threshold signal varying with the average input level of the noise detector.
Such a noise detector is disclosed in the United Kingdom Patent Specification No. 2,113,047.
In this patent specification, the prior art noise detector is described for use in a radio receiver whose RF-input signal is applied to the signal input of the signal comparison circuit by an RF-amplifier. The threshold signal at the threshold input of the signal comparison circuit varies, with a certain degree of dependence with the average IF-signal level, through which a noise selection which depends on the average input level or a field strength-dependent noise selection is realized. For that purpose, this threshold signal is supplied from the amplitude detection circuit which is coupled to the IF-receiving section.
In the event of large field strength variations, as occurs, for example, with mobile reception, non-linearities may occur in the prior art noise detector at amplification and/or amplitude detection of signals to be applied to the inputs of the signal comparison circuit. As a result the threshold signal does not vary everywhere within the input dynamic range with the same degree of dependence where the average input level and an unambiguous noise selection criterion is not possible. In addition, for a correct comparison of signals which may vary over a wide amplitude range, the signal comparison circuit must satisfy high requirements for accuracy and linearity.